1. Field of the Invention
The present invention relates to a camera provided with an image stabilization function.
2. Description of the Related Art
In recent years, the evolution of video apparatus, such as video cameras, has been remarkable, and automation and a greater number of functions have been realized in various aspects of the video apparatus. One recent example is the realization of a video camera provided with an image stabilization device which enables good photography by correcting a shake of an image due to a vibration of the camera. FIG. 1 is a block diagram of one example of a camera provided with an optical image stabilization function.
The example shown in FIG. 1 includes a light deflecting lens (variable angle prism) 101 for optically correcting the displacement of an optical axis and performing an image stabilization operation. The structure of the light deflecting lens 101 is such that, for example, a space surrounded by two flat sheet glasses and a bellows is filled with a liquid essentially consisting of silicone. The light deflecting lens 101 is arranged to perform two-dimensional deflection of light by the respective two sheet glasses being pressed vertically and horizontally by corresponding actuators which will be described later.
The example shown in FIG. 1 also includes an angular-velocity sensor 102 for detecting a vertical vibration, an angular-velocity sensor 103 for detecting a horizontal vibration, an image stabilization control circuit 104 for correcting a shake of an image due to a vibration by controlling the light deflecting lens 101 on the basis of vibration information supplied from the angular-velocity sensors 102 and 103 and varying the direction of refraction of light, a driver 105 for outputting an output for driving the light deflecting lens 101 to vertically deflect light, an actuator 106 for driving the light deflecting lens 101 in accordance with the output of the driver 105, a driver 107 for generating an output for driving the light deflecting lens 101 to horizontally deflect light, an actuator 108 for driving the light deflecting lens 101 in accordance with the output of the driver 107, an apex angle sensor 109 for detecting the amount of vertical deflection of light by the light deflecting lens 101, and an apex angle sensor 110 for detecting the amount of horizontal deflection of light by the light deflecting lens 101.
The example shown in FIG. 1 also includes a focus adjusting lens 111, an image pickup element 112 such as a CCD, an amplifier 113, a high-pass filter 114 for extracting a high-frequency component which varies with the state of focus, from a video signal outputted from the image pickup element 112, a lens control circuit 115 for moving the focus adjusting lens 111 toward an in-focus position, i.e., so that the level of the high-frequency component can reach a maximum, in accordance with the state of the output signal of the high-pass filter 114, a motor driver 116, and an actuator 117.
FIG. 2 is a graph showing the output level of the high-pass filter 114 with respect to the position of the focus adjusting lens 111 in the arrangement shown in FIG. 1. As is known, the output signal level of the high-pass filter 114 has the feature that if the focus adjusting lens 111 is out of focus, the high-pass filter 114 outputs a low-level signal and, if the focus adjusting lens 111 is in focus, the high-pass filter 114 outputs a maximum-level signal. The lens control circuit 115 utilizes such a feature and performs so-called "hill climbing AF" control for driving the focus adjusting lens 111 so as to make the output signal level of the high-pass filter 114 closer to a maximum.
FIG. 3 is a flowchart showing a specific process of the hill climbing AF control. In the flowchart shown in FIG. 3, if the process is started in Step 401, the process waits for arrival of a vertical synchronizing signal in Step 402. This is because, during each vertical synchronizing period, an image signal for one picture can only be obtained from a video signal supplied from the image pickup element 112. If the arrival of the vertical synchronizing signal is confirmed in Step 402, the output signal level of the high-pass filter 114 at that time is, in Step 403, inputted into the lens control circuit 115 as a focus voltage value indicative of the state of focus. In Step 404, on the basis of the inputted focus voltage value (by using a past voltage value as required), the lens control circuit 115 determines whether the focus adjusting lens 111 is currently located in an in-focus position. In the case of a largely out-of-focus state in which the focus adjusting lens 111 is entirely away from the in-focus position, the process proceeds to Step 406 through Step 405. In Step 406, the lens control circuit 115 makes an analysis to detect whether an in-focus point is present on a infinity-distance side or a closest-distance side, and drives the focus adjusting lens 111 to cause it to move at a high speed toward the side detected through the analysis. If it is determined in Step 408 that the focus adjusting lens 111 is not largely out of focus and is present in the vicinity of the in-focus point, the lens control circuit 115 executes the processing of Step 409 to confirm the direction in which the focus adjusting lens 111 is to be driven. In Step 410, the focus adjusting lens 111 is driven at a low speed. The reason why the driving speed of the focus adjusting lens 111 is lowered in the vicinity of the in-focus point is to minimize the amount by which the focus adjusting lens 111 overruns the in-focus point.
If it is determined that the current position of the focus adjusting lens 111 coincides with the in-focus point, i.e., if the result of the decision made in Step 405 is "NO" and the result of the decision made in Step 408 is "NO" and the result of the decision made in Step 411 is "YES", the focus adjusting lens 111 is made to stop in Step 412, and the process returns to Step 402. If it is not determined in Step 411 that the current position of the focus adjusting lens 111 coincides with the in-focus point, this indicates that it may be necessary to again perform a focusing operation because of, for example, the movement of a subject. Accordingly, the process proceeds to Step 413, in which it is determined whether it is necessary to restart driving of the focus adjusting lens 111. If it is necessary to restart driving of the focus adjusting lens 111, i.e., if the result of the decision made in Step 413 is "YES", the processing of Step 414 is executed to confirm the direction in which the focus adjusting lens 111 is to be driven. In Step 415, the lens control circuit 115 selects a driving speed suitable for the restarting. Then, the process returns to Step 402.
Since the image stabilization device and an automatic focus adjusting device are disposed as control systems independent of each other, the image stabilization device continues its image stabilization operation during the above-described focusing operation.
In the above-described related-art example, however, since the light deflecting lens 101 performs the image stabilization operation at all times, if the driving direction of the focus adjusting lens 111 is to be determined when the focus adjusting lens 111 is largely out of focus and the focus voltage value is considerably low, the focus voltage value may vary owing to a periodical, light deflecting action, for a cause other than the movement of the focus adjusting lens 111, so that an erroneous decision may be made as to the driving direction.
During the largely out-of-focus state, since the contour of an image is entirely unsharp, the influence of an unnecessary vibration, such as a vibration of a camera, upon photography is extremely small and the effect of the image stabilization operation is low. In addition, in the case of an image-stabilization camera integrated type video tape recorder, the possibility that a photographer starts photography during the largely out-of-focus state is comparatively low. For these reasons, the execution of the image stabilization operation during the largely out-of-focus state offers many demerits to the processing of the automatic focus adjusting device or the consumption of a power source such as a battery.